FIELD OF THE INVENTION
This invention relates generally to trolley systems typical of those used in subterranean mines. More specifically, this invention relates to ground fault detection on such trolley systems.
Many mine fires are started by electrical trolley wires because the conventional power circuit breaker overcurrent trip cannot discriminate between the high normal load currents and the smaller resistance ground fault currents. Because conventional circuit breakers must be set at such a level that they will not trip for normal load transients, many ground faults will not be detected. Rate of rise detection schemes which have been successfully applied on 600 volt trolley systems are ineffective on 300 volt trolleys because the inductance of the large 300 volt locomotive motors is less than that of the trolley wires themselves. As a result, a large locomotive, in starting a train near a power substation on a 250-300 volt dc system, can have a rate of rise of current (di/dt) much larger than the rate of rise due to a distance ground fault.
Typically, an underground trolley system includes a plurality of power substations spaced along the trolley line. Any short circuit between the line and ground produces a rapidly rising fault current.
The highest fault currents occur in the vicinity of the power substations. Trolley wires, like transmission lines have distributed reactance (primarily resistance and inductance). Therefore, ground faults and normal loads appearing on the line between two power substations will have greater impact at a particular substation if the fault or load is near that substation. In fact, normal loads at points near a substation can cause a higher current rate of rise at that substation than a ground fault further removed from that substation. The relatively low cable and trolley wire resistance near power substations permits high peak fault circuit currents, and the associated lower inductance enables the short circuit currents to build up much more rapidly than a fault on the section at a point midrange between the two substations. It is therefore important to detect fault conditions quickly at the points near the substations if the high fault currents and the associated high energy into the fault is to be controlled. FIG. 1 shows the current rate of rise as a function of the distance from the substations for resistance faults and for locomotives. As can be seen, the maximum rate of change of current for a large locomotive operating close-in to either of the substations can exceed that of a resistance fault in a locaton midrange between the two substations. It is also apparent from the figure that the rate of rise of a resistance ground fault close-in to a substation can greatly exceed that of even the largest locomotives.
Due to the fact that resistance ground fault currents may be as low as a few hundred amperes on a 300 volt dc trolley section, conventional circuit breakers set at several thousand amperes are not effective in protecting the system against the hazards associated with these ground faults. If the breakers are set at low current level to trip for ground faults, there will be many false alarm shut-downs resulting from high locomotive start-up currents. Although current rate of rise sensors can be useful on 600 v. trolley lines; they are not effective on 300 v. lines because the inductance of the locomotive motors is less than that of the trolley line. Start-up di/dt from close-in locomotives can be much greater than the di/dt for a distant ground fault.